While optical fiber transmission systems have numerous advantages, they also have several problematic aspects. For example, (1) the transmitter power is subject to high manufacturing tolerance; (2) the transmitter power decreases with time (depending on wavelength, from scarcely detectable to very strong); (3) optical fibers made from plastic age very quickly, especially when exposed to strong temperature fluctuations; (4) the surface of the contact points of the optical fibers is scratched by frequent changing of the plugs, which additionally increases the attenuation; and (5) the sensitivity of the receivers is likewise subject to high manufacturing tolerance. One of the worst problems is posed by aging and the decreasing transmitter power. The failure of an optical fiber link, for example owing to high attenuation of the optical fiber, can cripple entire systems.
Several different methods have been established to monitor the health of an optical fiber transmission system to detect such problems. However, current methods are extremely complicated, invasive, and often involve the disruption of the communication operation. For example, one method involves mechanically forcing and critically aligning a bare (uncladded) portion of an optical fiber against an unclad portion of the operational fiber in order to extract the information by evanescent wave coupling (i.e., the process by which electromagnetic waves are transmitted from one medium to another by means of the evanescent, exponentially decaying electromagnetic field). Another method involves bending the operational fiber enough to cause the transmitted information carrying light wave to fall into the escape cone of the fiber so that it can be retrieved by a diagnostic sensor.
As such, it would be advantageous to have a non-invasive fiber-optic sensor for performing real-time diagnostics on signal and information transmission with a minimum of disruption and complexity.